The Soil Moisture Active Passive (SMAP) mission will have the first L-band radar/radiometer sensor suite dedicated to global measurements of soil moisture. For the radar sensor, the requirements for achieving high backscatter measurement accuracy from low-Earth orbit present a unique design challenge in the presence of terrestrial radio frequency interference (RFI). The SMAP radar shares the same 1,215 to 1,300 MHz spectrum used by high-power ground-based transmitters like air-route and defense surveillance radars, which can generate strong interference in a conventional fixed-frequency spaceborne radar. The noisy ground environment motivated the development of a frequency-hopping (self-tuning) feature in the radar design. As the SMAP spacecraft orbits across various regions of the Earth, the radar continually adjusts its RF operating frequency to quieter areas of the spectrum for improved fidelity in soil-moisture science data observations.
A “tunable LO” (local oscillator) receiver architecture has been developed with narrowband analog/IF filtering to significantly reduce the SMAP radar’s susceptibility to RFI. A key advantage of the tunable LO scheme is the excellent selectivity of the IF filter stage, which blocks out-of-band noise before the A/D (analog-to-digital) conversion stage. A collateral benefit of the tunable LO approach is that the smaller receiver system bandwidth relaxes requirements on A/D sampling speed. Lower-speed A/D converters typically operate at lower power consumption and over a wider dynamic range.
During performance testing of the flight radar, it was verified that the 0.4 dB RFI error budget was satisfied with 0.2-dB margin. The SMAP radar is designed with RFI mitigation capabilities that include:
- dynamically tunable operating frequency, to allow the instrument to frequency-hop around noisier parts of the spectrum, and over particular regions of the Earth;
- sharp RF/digital receiver filtering with a high degree of selectivity for rejecting out-of-band interference (80+ dB rejection); and
- radar telemetry for flagging range lines contaminated by RFI, so that this bad data can be removed in science data post-processing on the ground.
The radar design uses a hybrid digital-analog approach to update radar operating frequencies dynamically in orbit. On the transmit leg, direct-digital synthesis is used to generate the pair of tunable frequencies for V- and H-polarized, 1-MHz chirp pulses; the fixed-LO upconverter and high-power amplifier are wideband (>80 MHz) to accommodate the range of chirped RF frequencies over 1,217 to 1,298 MHz, adjustable in 1.25-MHz steps. Each receive leg uses a single-stage heterodyne downconverter with tunable LO and FPGA-based back-end digital processor.
This work was done by Mark A. Fischman, Harry S. Figueroa, Kayla Nguyen, Andrew C. Berkun, Charles T-C Le, Mimi Paller, and Gerald J. Walsh of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49415
This Brief includes a Technical Support Package (TSP).
An Earth-Observing, Frequency-Agile Radar Receiver for RFI Mitigation
(reference NPO49415) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA’s Jet Propulsion Laboratory (JPL) detailing an Earth-Observing, Frequency-Agile Radar Receiver designed for Radio Frequency Interference (RFI) Mitigation. This technology is part of NASA's efforts to enhance the capabilities of remote sensing instruments, particularly for soil moisture observation and other Earth science applications.
The radar receiver is characterized by its frequency-agile design, which allows it to adapt to varying environmental conditions and mitigate interference from high-power L-band ground transmitters, such as those used in air and defense surveillance. This adaptability is crucial for ensuring the accuracy and reliability of data collected from space, especially in low-Earth orbit where RFI can significantly impact measurements.
Key features of the radar system include a hybrid Synthetic Aperture Radar (SAR) and scatterometer operating concept. The system is capable of collecting high-resolution phase-coherent SAR data and low-resolution echo power detection data, providing comprehensive backscatter measurements across various azimuth angles throughout its orbit. The radar parameters are updated every 4.6 seconds during beam revolutions, allowing for real-time adjustments to the Pulse Repetition Interval (PRI) and operating frequencies.
The document outlines built-in test functions for the radar receiver, including gain calibration between echo and noise-only channels, Doppler shift correction, and noise diode calibration for the radiometer. These functions are essential for maintaining the health and accuracy of the radar system, ensuring that it can effectively monitor soil moisture and other environmental variables.
Additionally, the document emphasizes the importance of reducing the net downlink data rate, which is critical for efficient data transmission back to Earth. The design also incorporates digital monitoring patterns to flag Single Event Upsets (SEUs) in the digital electronics, enhancing the reliability of the system.
Overall, this Technical Support Package serves as a comprehensive overview of the innovative radar technology developed by JPL, highlighting its potential applications in Earth observation and the challenges it addresses in terms of RFI mitigation and calibration stability. The information is intended to support the broader goals of NASA's Commercial Technology Program, promoting the transfer of aerospace-related developments for wider technological and scientific applications.
